Rubber compositions for a tire are required to have properties such as high hardness (E*), low tan δ at 30° C. to 70° C., and excellent elongation at break.
Methods for achieving high hardness include adding a compound such as sulfur, a vulcanization accelerator, a resorcinol resin, or a polyethylene resin, and increasing the amount of filler such as carbon black or silica. However, achieving both high hardness (E*) and good elongation at break (EB) is difficult with these methods.
Generally, a rubber composition having an elongation at break of less than 100%, even if having high hardness, cannot be used as a rubber composition for a tire which is required to have durability. For example, the elongation at break measured at room temperature (25° C.) is required to be at least 100% in the case of a rubber composition for a bead apex, at least 200% in the case of a rubber composition for a breaker (a rubber composition for a breaker topping), and at least 350% in the case of a rubber composition for a tread (cap tread or base tread) or a sidewall.
Both high hardness (E*) and not less than 100% of elongation at break (EB) can be achieved by addition of a phenol resin. This addition, however, is disadvantageously accompanied by an increase of tan δ, and thus it has been difficult to achieve high hardness (E*), low tan δ (good fuel economy), and good elongation at break in a balanced manner. For this reason, rubber compositions containing a phenol resin are seldom used for tire components other than bead apexes and breakers in the current state of the art.
Patent Documents 1 and 2 each involve a rubber composition containing a cresol resin as an alkylphenol resin. However, the cresol resins used in Patent Documents 1 and 2 are obtained by using only m-cresol, which is a 3-alkylphenol, as the alkylphenol component (monomer component); here, use of resins obtainable using monomer components other than 3-alkylphenols (e.g. 2-alkylphenols, 4-alkylphenols) and the amount of free monomer components contained (remaining) in alkylphenol resins have not been studied in detail. Therefore, these rubber compositions still leave something to be desired in terms of achieving high hardness (good handling stability), low tan δ (good fuel economy), good elongation at break, and good steering response in a balanced manner while maintaining good extrusion processability.
Patent Document 1: JP 2008-31427 A
Patent Document 2: JP 2010-52724 A